Editor's Note: This article was updated on 11 September 2005 to give a more complete account of previous work on CaC6 than the version that was published on 30 August 2005.
Bulk quantities of a new carbon-based superconductor have been made by physicists in France. Known as CaC6, the material contains "foreign" calcium atoms inserted between hexagonal two-dimensional sheets of graphite. Earlier this year Tom Weller and co-workers at University College London (UCL) and Cambridge University discovered that CaC6 becomes a superconductor when cooled to below 11.5 Kelvin. Now Nicolas Emery of the University Henri Poincaré in Nancy and colleagues at the University of Paris 6 & 7 have demonstrated how to make bulk quantities of the material.
Superconductivity is the complete absence of electrical resistance and is observed in certain materials when they are cooled below a superconducting transition temperature (Tc). Physicists agree that superconductivity relies on getting electrons to overcome their mutual Coulomb repulsion and form “Cooper pairs”. In the Bardeen-Cooper-Schrieffer (BCS) theory of low-temperature superconductivity, the electrons are held together because of their interactions with phonons – lattice vibrations in the material.
CaC6 is an example of a “graphite intercalated compound” – a class of electronic material in which foreign (or guest) atoms, such as calcium, sodium and potassium, are inserted into graphite. These materials consist of two-dimensional layers of graphite with layers of guest atoms in between. Graphite is a semi-metal, which means that electrons accepted or donated by the foreign atoms modify the properties of the graphite, making the final material metallic.
The first superconducting graphite-doped compound – potassium carbide (KC8) – was discovered 40 years ago and had a Tc of just 0.14 Kelvin. Earlier this year, researchers at UCL and Cambridge showed that ytterbium carbide (YbC6) also becomes superconducting at 6.5 Kelvin and obtained evidence that CaC6 superconducts at a temperature of 11.5 Kelvin, which is the highest Tc observed in a graphite intercalated compound so far (cond-mat/0503570). However, in that experiment the CaC6 formed in micron-thick layers on the surfaces and edges of the graphite host. Emery and co-workers have now developed a new method to make high quality bulk samples of CaC6. This development has resulted in sharpening of the superconducting transition.
The technique involves heating pyrolytic graphite with a molten lithium-calcium alloy at 350°C under an atmosphere of argon for 10 days (Phys. Rev. Lett. 95 087003; also available as cond-mat/0506093). Using X-ray diffraction, the physicists showed that CaC6 is the only member of the MC6 family (where M is a metal atom) to have rhombohedral symmetry — the others are hexagonal. They also found a sharp drop in the magnetisation of the material below 11.5 Kelvin.
According to calculations by Matteo Calandra and Francesco Mauri from the University of Paris 6, who collaborated with Emery’s team, superconductivity in CaC6 is due to an electron-phonon mechanism (cond-mat/0506082). The charge carriers in the material are mostly electrons in the “Fermi surface” of calcium that couple to vibrations of the carbon atoms moving perpendicular to the graphite layers and calcium atoms moving parellel to the layers. Furthermore, the results suggest that this could be a general mechanism for all graphite intercalation compounds.
In an earlier theoretical paper Gabor Csányi and co-workers at Cambridge wrote that they had found “a striking correlation” between superconductivity in YbC6, CaC6 and similar materials and the occupation by doped electrons of an interlayer energy band that lies between the graphite sheets (cond-mat/0503569). Csányi and co-workers conclude that this “suggests the possibility of a pairing mechanism linked not to lattice but to soft charge fluctuations”.
